This invention pertains to apparatus for making dental radiographs. The invention relates particularly to an electronically controlled system for delivering constant and precisely regulated d-c anode voltage and filament current to an x-ray tube to permit governing radiographic exposures exclusively by the user's choice of the exposure time interval.
Conventional dental x-ray apparatus comprises an oil-filled x-ray tube casing or tube head which is mounted on a pantograph arm or the like to permit the dentist to locate the x-ray tube adjacent a patient's head for making a radiograph. The tube head must be counterpoised to constrain it to remain where the user locates it. Consequently, it is desirable that the tube head be as small and lightweight as possible.
Traditional dental x-ray tube power supplies for controlling the x-ray exposure factors, that is, the anode voltage and current supplied to the x-ray tube and the exposure time, involve manually setting or regulating the voltage and stepping this voltage up at power line frequency to obtain the desired anode voltage with a transformer in the tube casing or head where it usually undergoes half-wave rectification before being applied between the cathode filament and the anode of the x-ray tube. The x-ray tube filament current which controls the temperature and electron emissivity of the filament and, hence, enables control over the x-ray exposure intensity, is also supplied through a transformer in the tube head. Since the transformers which supply the anode voltage and filament current operate at power line frequency, which is typically 50 or 60 Hz, their transformer cores must necessarily use a lot of core steel to minimize magnetic losses. This results in the x-ray tube head being heavy and bulky and causes some other problems which are well known.
The x-ray tube factor controls used in traditional dental x-ray apparatus have also been unduly bulky. One reason is that they operate at power line frequencies throughout. Another is that they employ large autotransformers to provide a variety of primary voltages to the high voltage anode transformer. Still another is that they are adapted to provide a variety of x-ray tube currents and x-ray exposure intervals. A basic problem that results from all this is that the electric power requirements become very high and the system becomes very susceptible to power line voltage fluctuations.
In addition, the electric power conversion and control systems used in prior dental x-ray apparatus exhibit low electrical efficiency. The input power to the apparatus is high compared with the useful output power delivered to the x-ray tube. The input and output power difference is wasted and only produces heat which is another problem the designer must cope with. The poor efficiency precludes dental x-ray power supply and control designs which can be operated from a low power supply circuit in a building.
Moreover, prior designs have resorted to bulky and expensive means to safeguard the user and patients against the hazard of electric shock which is always present in high voltage equipment.